Conventional cardiac pacemakers and defibrillators are generally equipped with a generator for electrical stimulation that is implanted in a patient's body distally from the heart for cardiac pacing and defibrillation.
Conventional cardiac pacemakers may typically be designed as a single chamber or a dual chamber device. The single chamber device is capable of sensing and pacing in one cardiac chamber, either the atrium or the ventricle. Dual chamber devices have the capability of sensing and pacing in both (atrium and ventricle) cardiac chambers.
In addition to the right atrium pacing and the ventricle pacing, the left ventricle (by way of the cardiac veins or bi-ventricular pacing) provides a physiologic and synchronous cardiac contraction which would improve cardiac function.
Modes of pacing operations may be identified through letter sequences which include the following identifications: VDD, DVI, VVI, and DDD. The first letter of the mode identification indicates the cardiac chamber being paced. The second letter indicates the cardiac chamber being sensed. The third letter indicates the responses being inhibited or triggered. A fourth letter “R” may be added to the mode identifications which denotes a rate responsive pacing to match patient's activities.
The conventional cardiac pacemakers are equipped with elongated flexible pacemaker leads (cardiac leads) which are typically connected to the generator. The cardiac leads are conventionally configured with tubular electrically insulated sleeve structures that are inserted into the body through an incision overlying the veins and leading to the heart chambers where a distal end of each lead is lodged. In such cases, the distal end of the cardiac lead is connected to a tubular tip electrode which has an increased diameter forming an annular shoulder against which the distal end of the tubular sleeve abuts.
Conventionally, two types of cardiac leads are used in the pacemaker systems, i.e., the uni-polar leads and the bipolar leads. The uni-polar cardiac lead operates with a single conductor coil, and typically includes a cathode (or negative pole) at the distal tip, and an anode (or positive pole) defined by the housing of the stimulator. Electric current returns to the anode via the body tissue as a current path.
The bipolar cardiac lead has two conductor coils, the distal tip forming the cathode, and an annular (or ring) electrode located a few millimeters proximal to the distal tip. High voltage defibrillation is delivered by one or two shocking coils that are inserted intravenously.
Pacemaker leads (referred to herein intermittently also as cardiac leads) are generally adapted for placement in the ventricle and atrium. In order to provide permanent pacing and to avoid the pacemaker lead dislodgement, various techniques have been used for anchoring the pacemaker leads to the endocardium (which is the inner lining of the heart chambers). Pacing the right ventricular outflow is desirable due to antegrade conduction, compared to retrograde conduction that is inherent with apical pacing.
However, right ventricular outflow pacing may not be feasible using conventional transvenous electrodes due to the difficulty of securing a stable position and the high risk of dislodgment. Conventional right ventricular apical pacing alters the normal synchronization of different heart chambers, and may adversely influence ventricular function, leading to heart failure.
In addition, right ventricular leads in conventional pacemakers, when implanted, disadvantageously cross the tricuspid valve which is located on the right dorsal side of the heart between the right atrium and the right ventricle. Such leads may cause unwanted tricuspid regurgitation by interfering with tricuspid valve closing during heart contraction which may interfere with the right ventricular function.
Bi-ventricular pacing (or resynchronization) requires the placement of electrodes within the venous system of the heart. However, other than lodging the tip of the lead into the distal coronary vein, there has been found no safe anchoring mechanism to maintain the lead from dislodging. Screw-in anchors may be applied to the myocardium, but cannot be utilized in vascular structures due to the risk of endothelial damage and hemorrhage.
Additionally, the optimum lodging site may not be the ideal pacing location for effective myocardial stimulation.